This invention relates to an erasable ink composition and to a marking instrument, e.g., a ball-point pen, containing the composition. More particularly, this invention relates to an erasable ink composition containing an aqueous dispersion of a polymer-encapsulated colorant.
Numerous erasable ink compositions are known, e.g., those described in U.S. Pat. Nos. 3,834,823, 3,875,105, 3,949,132, 4,097,290, 4,212,676, 4,227,930, 4,256,494, 4,297,260, 4,329,262, 4,329,264, 4,349,639, 4,357,431, 4,367,966, 4,368,076, 4,379,867, 4,389,499, 4,390,646, 4,391,927, 4,407,985, 4,410,643, 4,419,464, 4,441,928, 4,509,982, 4,525,216, 4,557,618, 4,578,117, 4,596,846, 4,606,769, 4,629,748, 4,687,791, 4,721,739, 4,738,725, 4,760,104, 4,786,198, 4,830,670, 4,954,174, 4,960,464, 5,004,763, 5,024,898, 5,037,702, 5,082,495, 5,114,479, 5,120,359, 5,160,369 and 5,217,255. These inks are formulated by mixing a colorant (taken herein to also include "pigment", "dye", "chromophore" and other terms of similar meaning) with a variety of polymer and liquid carrier/solvent combinations thereby forming a flowable colorant/polymer matrix. The polymer component is chosen for its film forming properties and its ability to be readily removed from the substrate to which it is applied, e.g., cellulosic paper, through the abrasive action of an eraser. However, a common problem with these erasable ink compositions concerns the residual colorant which remains after erasure. Incomplete erasure may be attributed to inadequate removal of the colorant/polymer matrix from the paper substrate and/or migration of colorant into the pores of the substrate.